1. Field of the Invention
The present invention relates to an improvement of a magnetic recording medium, and more particularly to an improvement of the backcoat layer formed on the reverse side of the support to the magnetic layer of a magnetic recording medium.
2. Description of the Prior Art
Generally, a magnetic recording medium comprises a support such as polyethylene terephthalate film and a magnetic material layer formed by coating and drying thereon a magnetic coating material obtained by mixing and dispersing a spiculate magnetic powdery material into a binder.
Such a magnetic recording medium, in the form of a magnetic tape, magnetic disk, magnetic sheet, floppy disk, or the like, is widely used in the fields of audio, video, computer, etc. Particularly, with the remarkable spread of video cassettes in these days, just as those audio cassette tapes improved on the tape thickness to be much smaller have been produced and marketed in order to meet the need for long-time recording and reproducing, the video cassette tape product also has been necessitated to be improved so thinner as to be capable of long-time picture recording and reproducing.
The factor that determines the characteristics of these magnetic recording media is, of course, the nature of the magnetic layer on the support, but that of the reverse side of the support has an important effect on the use as well as on the magnetic recording characteristics. For this reason, with the thinning of the tape, in order to reinforce its mechanical strength a backcoat layer is provided on the reverse side of the support to its magnetic layer-formed side.
For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 176535/1982 proposes, in order to smooth the magnetic layer-forming surface of the support and the surface of the magnetic layer, to provide a backcoat layer containing silicon-modified polyurethane on the reverse side of the support.
However, the backcoat layer consisting of a binder alone is disadvantageous because it has as large a surface electric resistance as to cause static electricity during the running of the tape to adsorb dust to increase dropout, and also has the disadvantage that the backcoat layer and the magnetic layer, during the storage of the tape wound in the reel form, tend to stick to each other. And if the surface of the magnetic layer is treated to be smooth like a mirror, the contacting area between the tape surface or between the tape and other objects such as guide rods becomes wider.
Thus, the frictional resistance increases, and therefore the run of the tape becomes unstable (because, during the running, the tape comes into contact with guide rollers or becomes elongatively deformed.).
To solve the problem of such running instability resulting from the surface smoothing of the magnetic layer, attempts are made to roughen the surface of the backcoat layer by use of a binder containing carbon black, titanium oxide powdery material, or the like, but any satisfactory running stability and electromagnetic conversion characteristic are still not obtained. If a backcoat layer is formed by the above method, the pattern of the roughened surface of the backcoat layer is transferred onto the smooth surface of the magnetic layer at the time of winding the tape, the surface smoothness of the magnetic layer is reduced, and the electromagnetic conversion characteristic, chroma S/N and video S/N are deteriorated.
Further, the electromagnetic recording medium, if the surface roughness of the backcoat layer is not appropriate, can cause cinching or loosening trouble (a reel cinching or loosening phenomenon caused when the tape abruptly stops).
In order to solve the disadvantageous problem of the above-mentioned transfer of the pattern of the rough surface of the backcoat layer formed by use of a binder containing such a powdery material as carbon black, titanium oxide, etc., onto the smooth surface of the magnetic layer at the time of tape winding, Japanese Patent O.P.I. Publication No. 150133/1982 proposes the incorporation of as lubricants a fluorine compound, alkoxysilane, and silane fluoride into the backcoat layer.
However, the incorporation of such low-molecular lubricants into the backcoat layer often causes the lubricants to deposit, depending on the kinds or adding quantities thereof or due to the change in environmental conditions, so that a bleed-out phenomenon tends to occur on the surface of the backcoat layer and causes the magnetic layer and backcoat layer to stick to each other.
For solution to the problem of the above-described transfer of the pattern of the surface roughness of the backcoat layer resulting from the incorporation of such a powdery material as carbon black, titanium oxide, etc., onto the surface of the magnetic layer, Japanese Patent O.P.I. Publication No. 208636/1982 proposes the selection of an appropriate graininess and content range of such a nonmagnetic powdery material as carbon black, titanium oxide, etc., to be contained in the backcoat layer and the dispersion of such a powdery material into a binder comprised of nitrocellulose, vinyl chloride-vinyl acetate copolymer, polyurethane, and isocyanate compound.
These methods using a nonmagnetic powdery material and lubricants in the backcoat layer can surely reduce the foregoing disadvantages to a considerable extent due to their synergistic effect, but the characteristic (such as coefficient of friction) of the binder itself is not satisfactory for the running stability, so that they are not yet considered sufficient as the characteristic necessary for the backcoat layer.
In view of the above conditions, as a result of our investigation made with respect to a magnetic recording medium comprising a support having on one side thereof a magnetic layer and on the other a backcoat layer, it has now been found that:
(a) the method of incorporating nonmagnetic powdery matereals into the backcoat layer will not display its effects (reinforcement of the tape, removal of the winding disarray resulting from insufficient ejection of the air involved in between tape winds at the time of high-speed tape winding, prevention of permanent deformation resulting from the winding disarray, prevention of dropout and the lowering of S/N) unless the surface is appropriately coarsely roughed,
(b) if the surface of the backcoat layer is sufficiently roughed, the pattern of the rough surface is transferred onto the surface of the magnetic layer, thus deteriorating the characteristics of the tape, and
(c) silicon-modified polyurethane in itself has lubricity, and the incorporation of it along with nonmagnetic powdery materials and particularly with an appropriate BET value-having carbon black into the backcoat layer can maintain the surface of the backcoat layer comparatively flat and smooth, reducing the transfer of the pattern onto the magnetic layer, and enables the backcoat layer to retain its appropriate roughness, thus preventing possible winding disarray of the tape at the time of reel winding, and further the incorporation of the polyurethane lowers the surface resistance of the backcoat layer to thereby prevent the adsorption of dust by the accumulation of static electricity, and also lowers the light transmittance of the tape, so that the adding quantity of carbon black to the magnetic layer can be reduced, the mechanical-physical properties can be improved, and the sticking between the backcoat and magnetic layers can be prevented.
Thus, we have reached the present invention.